In a thermo-photo-voltaic device, a thermal heat source is mated with a III-V photovoltaic cell. Thermal photons are radiated from the heat source and those with energy greater than the bandgap of the photovoltaic cell will generate electricity in the same manner as the more familiar solar cell. A micron-gap thermo photovoltaic device operates in similar fashion except the heat source and the photovoltaic device are separated by a gap of less than one micron. See the paper entitled “Micron-gap ThermoPhoto Voltaic (MTPV)”, DiMatteo et al., proceedings of the Fifth TPV Conference, 2002 incorporated herein by this reference.
For experimental purposes, the heat source is a heater chip which is heated with an on-board electrical resistance heater but in application the heat could be derived from solar energy, combustion of fuel, or the like. In one application, the system consists of a photovoltaic cell, a heat source substrate and the heat source itself. The function of the heat source substrate is to form a high temperature radiating surface at a distance less than one micron from the photovoltaic surface. The thermal energy is applied to the back of the heat source substrate by convection or radiation. Various researchers have been looking at conventional TPV technology for about 50 years. The applicants' approach is unique is that the heat source is spaced from the photovoltaic cell by a gap of less than a micron. This micron gap spacing gets around Plank's law and allows the system to function as though the black body emissivity were greater than one. A factor of ten has been predicted and observed for 0.1 micron gaps. That is, the photocurrent obtained with a 0.1 micron spacing is about ten times that obtained when the heater chip is moved about two microns or more away from the photovoltaic cell. This is a very large effect and has the potential to revolutionize the field of thermo-photo-voltaics. For a given temperature of operation, one can decrease the size of the overall system and still achieve the same power or one could operate at a lower temperature easing materials problems and helping to make the use of thermo-photo-voltaics more practical.
To avoid thermal shorting, the system operates in a vacuum and spacers of silicon dioxide are employed to set the gap between the heater source and the photovoltaic cell in a manner which minimizes the heat transfer through the spacers. Phonons or non radiated energy carriers are a source of inefficiency as they transfer energy from the source but do not have the individual potential energy to excite electrons across the bandgap.
As described in the above referenced paper, the previous method of forming the spacers between the heat source and the photovoltaic cell was to grow a thick oxide on the heater chip and pattern the spacers to be about six microns in diameter. One disadvantage of this former method is that the spacers permit too large a heat loss from the heater. Despite the fact that the diameter of the spacers is small and the thermal resistance of silicon dioxide is greater than that of silicon, about 30% to 50% of the parasitic heat loss from the heater is due to these spacers. Typically, spacers are used in the four corners of the heater and there are at least three spacers per location. A high heat loss means that the efficiency of conversion of heat to electricity is low and also the cooling requirements on the photovoltaic cell are increased.
Another disadvantage is that the spacers can cause damage to the photovoltaic cell surface. Since the etching of the silicon dioxide is isotropic, the spacers are etched inwards at the surface. The base of the spacers is the mask dimension, about six microns. However, the tops of the spacers are narrowed to about two microns. The small size of the tips of the spacers causes the spacers to dig into certain photovoltaic cells because the material is relatively soft. Also, pressure applied to the heater/photovoltaic cell assembly can break the spacers causing debris and limiting the effectiveness of the spacers.
Another disadvantage relates to the eventual use of micron gap thermo-photo-voltaic devices for generating power which requires large area devices. One method of building a large area working system of heaters on the photovoltaic surface is to braze individual heater chips down on the photovoltaic cell creating a “tiled” surface. See U.S. Pat. No. 6,232,546 incorporated herein by this reference. A single large heater chip cannot be used because the heater is operated at about 1000° C. and the photovoltaic cell must be kept at room temperature to function effectively as a collector of photons and a generator of electrons. The difference in thermal expansion between the heater and the photovoltaic cell as the heater chip is heated from room temperature to 1000° C. can break the spacers or distort the geometry during the temperature excursion if there is a rigid attachment. This necessitates the use of tiling and the issue now becomes how to attach the heater tiles to the photovoltaic cell without creating a thermal short. With the tips of the spacers at 2 to 6 microns in diameter, there may not be enough room for the deposition of sufficient braze material to make this a practical method of attaching the heater chips to the photovoltaic cell.